1. Field of the Invention
The present invention relates generally to a printing mechanism having a unique motion transmitter, and more particularly pertains to a printing mechanism adapted to be powered or actuated with a reciprocating movement by a continuously rotatable driving drum.
2. Discussion of the Prior Art
In greater detail, the subject invention is concerned with the operation of a motion transmitter in which an electrically conductive band is positioned around a rotatable driving drum, and is movable lengthwise by the rotating drum because of an electrostatic force generated between the band and the rotating drum by the application of a voltage pulse therebetween. A motion transmitter of this nature utilizes the well-known Johnsen-Rahbek effect, the theoretical and practical considerations of which are carefully considered in a paper by Ms. Stuckes under the title "Some Theoretical and Practical Considerations of the Johnsen-Rahbek Effect," Proceedings I. E. E. E., Vol. 103, Part B, No. 8, March 1956, pages 125 to 131. As a result of tests described in this paper, the conclusion was reached that an electrostatic motion transmitter involving a continuously rotatable driving member is not a practical arrangement, particularly because of the existence of problems in the areas of wear and heat generation, to which no adequate solution was foreseen.
The aforesaid problems have been overcome to a limited degree in several printing mechanisms proposed by the prior art. Previous designs for printing mechanisms using an electrostatic clutch principle based on the Johnsen-Rahbek effect have been operated in an active mode in which the clutch drives a print hammer to perform the printing operation during the electrostatic field charging portion of the cycle. A typical printing mechanism of this type utilizes a semiconductive coated drum rotated at a high speed and a steel band wrapped around the drum. One end of the band is attached to a spring, and the other end is secured to a print hammer. When a voltage pulse is applied to the steel band, the resultant electrical field generates an electrostatic force which holds the band against the drum. Rotation of the drum then carries the band and the hammer forward to perform a printing operation. Accordingly in these prior art printers, printing is accomplished when the clutch is initially electrostatically energized. During the idling portion of the cycle, the band slides continuously over the surface of the rotating drum. In these known mechanisms printing energy requirements have dictated that the hammer be actuated at a high speed, which in turn requires the drum to be rotated at a high speed. A typical drum circumferential speed is of the order of 150 inches per second. Accordingly the band constantly rubs or slides against the drum with a relatively high velocity, which results in rather severe wear conditions for the semiconductive coating on the drum and the encompassing band. Additionally a large initial tension on the band is required to maintain the band in good contact with the drum while it is rotating at high speeds, which again increases the resultant wear of the surfaces.
Foster U.S. Pat. No. 2,850,907, Foster U.S. Pat. No. 2,850,908 and Planer et al. U.S. Pat. No. 2,916,920 are illustrative of the aforementioned designs for printing mechanisms, and each has followed a slightly different approach to limiting wear problems therein.
Foster U.S. Pat. No. 2,850,907 discloses a motion transmitter for a printing mechanism having a continuously operable driving drum and a driven band member supported for relative movement along with the driving member, each having an electrically conductive face arranged to engage opposite sides of an intermediate member extending therebetween. The intermediate member is selected to have a dielectric constant and thickness such that energization of the printer by a current with a constant peak value causes clutching of the driven member to the driving member. The driven member is released from the driving member upon deenergization of the circuit by a shunt circuit connected across the conductive faces.
Foster U.S. Pat. No. 2,850,908 provides a motion transmitter for a printing mechanism having an electrically conductive band looped around a continuously rotatable driving member and in which an intermediate element is engaged and carried by the driving member for rotation therewith. The intermediate member has a pivoted lever with arms of unequal length disposed on opposite sides of the pivot, and the ends of the band are connected to each of the arms. Upon lengthwise movement of the band by the driving member, tension in the band portion connection to the shorter arm of the lever is relieved, thereby minimizing wear and heating at a position at which maximum friction would normally occur between the band and the intermediate element.
Planer et al. U.S. Pat. No. 2,916,920 illustrates a motion transmitter for a printing mechanism in which an electrically conductive band is looped around a continuously rotatable driving member, and in which an intermediate member is engaged by the band. An electrostatic force is developed between the band and the intermediate member, and is developed on a low friction track rotatable relative to the band. This produces a force in the band which is tangentially applied to a high friction track, also rotatable relative to the band, whereby the load applied to the band during a printing operation is accommodated by the high friction track.
In general, these prior art arrangements have still resulted in excessive wear of the relatively movable surfaces of the electrostatic printing mechanism.